In such methods and apparatuses, biopolymers, such as starches or proteins for example, are pressure-cooked in a few seconds, substantially by the action of shearing forces and expanded to a foamy structure. The term "cooking extrusion" is therefore also used.
Such a method for the extrusion of foodstuffs consisting of starch carriers and proteins is known from DE-OS No. 33 41 090 wherein a proportioned addition of water is effected by means of flow control devices at the beginning of the conveying path of the extrusion apparatus. In this case, however, the metered addition of water is effected exclusively from the point of view of adjusting a specific water content.
It is known from U.S. Pat. No. 3,711,296 to increase the natural water content of the raw material by an appropriate addition of water. By varying the speed of rotation of the screw of the extruder, an attempt is made to adjust pressure and temperature conditions under which a plastifying necessary for the extrusion takes place. In addition, it is known, per se, from this printed document to provide a steaming-out zone in the second half of the extruder or alternatively to this, an offtake along the extruder. This offtake is effected with the object of removing the water vapour given off during the plastifying, as completely as possible.
A basic problem with such cooking extrusion operations lies in the fact that, apart from the great fluctuations in the water content of the biological raw materials, these also occur in the fat and cellulose contents, as a result of which the quality of the product, for example hardness, density, water solubility and viscosity, colour and taste, is altered, while fluctuation in the proportioning of the individual components, during starting phenomena and in the temperature of the raw materials cause the same negative effect.